C# - Hashtable

The Hashtable is a non-generic collection that stores key-value pairs, similar to generic Dictionary<TKey, TValue> collection. It optimizes lookups by computing the hash code of each key and stores it in a different bucket internally and then matches the hash code of the specified key at the time of accessing values.

Hashtable Characteristics

  • Hashtable stores key-value pairs.
  • Comes under System.Collections namespace.
  • Implements IDictionary interface.
  • Keys must be unique and cannot be null.
  • Values can be null or duplicate.
  • Values can be accessed by passing associated key in the indexer e.g. myHashtable[key]
  • Elements are stored as DictionaryEntry objects.

Creating a Hashtable

The following example demonstrates creating a Hashtable and adding elements.

Example: Create and Add Elements
Hashtable numberNames = new Hashtable();
numberNames.Add(1,"One"); //adding a key/value using the Add() method
numberNames.Add(2,"Two");
numberNames.Add(3,"Three");

//The following throws run-time exception: key already added.
//numberNames.Add(3, "Three"); 

foreach(DictionaryEntry de in numberNames)
    Console.WriteLine("Key: {0}, Value: {1}", de.Key, de.Value);
		
//creating a Hashtable using collection-initializer syntax
var cities = new Hashtable(){
	{"UK", "London, Manchester, Birmingham"},
	{"USA", "Chicago, New York, Washington"},
	{"India", "Mumbai, New Delhi, Pune"}
};
		
foreach(DictionaryEntry de in cities)
    Console.WriteLine("Key: {0}, Value: {1}", de.Key, de.Value);
Try it

The Hashtable collection can include all the elements of Dictionary, as shown below.

Example: Add Dictionary in Hashtable
Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1, "one");
dict.Add(2, "two");
dict.Add(3, "three");

Hashtable ht = new Hashtable(dict);

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